Learn how to determine when you are the most fertile and likely to conceive.
By Dr. Nazaneen Homaifar, Medical Advisor, Natalist
You’ve made the exciting decision to try to have a baby! Now, you might be wondering how you should be timing sex to optimize your chances of conceiving. Knowing the basics of how a woman’s menstrual cycle works, and how to track it, will help you know when the “fertile window” is — aka, the most fruitful time for baby-making sex.
Every woman’s body is different, and conceiving can feel daunting and complicated at times. We’re here to provide reliable, no-nonsense information to help you make sense of the hormonal party going down each month.
Get in Sync
Did you know that most women are born with one to two million eggs in their ovaries? It’s their entire lifetime supply. These eggs are immature, and most of them will never have the chance to become fertilized. The way we talk about this lifetime supply can make it sound like you have two million chances to conceive, but it’s actually way less. Only about 300–500 of these eggs will mature over a woman’s life span.
Puberty not only marks an era of awkwardness and flailing with training bras for many of us, but also the time when certain hormones (we’ll get to which ones next) are released to help the eggs embark on this maturation journey. Herein lies the start of periods.
First, let’s get into the hormones that pull most of the strings: two of them are produced in the brain — follicle stimulating hormone (FSH) and luteinizing hormone (LH) — and two of them are produced in the ovaries — progesterone and estrogen. They each play a part in the regular drama that is a woman’s period.
On day one of a period (or menstrual cycle for the technical term), women are in what’s called the follicular phase of their cycle. It’s good to keep track of this moment on a calendar when trying to conceive — doctors will likely ask about a woman’s last menstrual cycle, and this is the date they’re looking for. This phase in the cycle is named for the follicle, a fluid-filled sac in the ovary that holds an immature egg.
What happens next is a cascade of activity you wouldn’t even know is happening: FSH gets the party started by telling the ovaries to begin to mature the tiny eggs they hold. As follicles grow, they make more estrogen, signaling to the uterus that it’s time to thicken its lining (the endometrium) and telling the brain it’s time to puppeteer the ovulation process. So at the same time that your ovary is going through the follicular phase, we name the phase of thickening of the uterine lining as the proliferative phase.
The estrogen-hyped brain releases LH, telling one of the ovaries it is time to release an egg (the ovaries usually take turns doing this — though the work isn’t always evenly split between the two). This is the main event: ovulation. A now-mature egg breaks out of its follicle and through the ovarian wall, journeying down the fallopian tube where it’ll wait for sperm to fertilize it.
Here’s where timing of intercourse becomes important: A person’s “fertile window” is clinically defined as the six day window that includes the five days before ovulation and the day of ovulation, and experts recommend having sex daily or every other day during this time, especiallythe 2 or 3 days right before you ovulate. This is when your chances of conceiving are the highest.
Meanwhile, the follicle transforms (seriously) into a little hormone factory called the corpus luteum, and it turns the uterus into a cozy home where a fertilized embryo can thrive by releasing progesterone to support the build-up of the uterine lining. In terms of the ovaries, this is called the luteal phase of your cycle. The transformation of the uterine lining from progesterone is referred to as the secretory phase.
If the egg on deck does meet a sperm and all other conditions are right, then hello, pregnancy! If it doesn’t, then a period ensues, as usual. This is because, in the absence of fertilization, the corpus luteum will stop hormone production and break down. No more hormones means no more uterine lining, so the lining sheds off and leaves the system — as a bloody mess, thank you very much!
Despite the requisite discomfort a menstrual cycle can bring, the female body is incredibly impressive in this way. During every cycle, it’s going through this laborious process to prepare the perfect environment just in case it receives and needs to nurture a fertilized embryo, whether or not a woman is actually interested in conceiving that month.
The average adult menstrual cycle lasts 28 to 35 days, with approximately 14 to 21 days in the follicular phase and 14 days in the luteal phase, but menstrual cycles between as few as 25 and 35 days still generally result in ovulation. There’s a lot of variability among women, and that’s okay.
How Do You Know You’re Ovulating?
Sure, you understand the different steps of the cycle, and it all looks great on paper, but how can you actually figure out when the fertile window is, other than doing some math?
Fortunately, there are several options — some are DIY, others are inexpensive tests.
DIY Method
- The DIY method for women is tracking cervical mucus using just their fingers. If the mucus is roughly the consistency and look of egg whites, that’s a good indicator that it’s an ideal time to have intentional, unprotected sex. This specific type of cervical mucus is ideal for helping sperm successfully journey to an egg.
Ovulation Predictor Kits
- There are also all sorts of tests for ovulation, and many work similarly to a pregnancy test by checking urine for certain hormones. In this case, the test is measuring LH levels. LH, as you’ll recall, gives ovaries the green light to release eggs every month. LH levels increase significantly about 12–36 hours before ovulation so an LH surge is a good indicator of the fertile window, and this is another way to know it’s the ideal time to have baby-making sex.
- Plenty of research supports the accuracy of these LH tests, but it’s good to know that they can occasionally show false positives. False positives can be especially common for women with medical conditions where LH might be abnormally elevated, like polycystic ovary syndrome (PCOS) or primary ovarian insufficiency.
- But what if you’re a woman whose cycle length is erratic? It can be difficult to know when to test for an LH surge or cervical mucus. If your cycle length is different month to month by more than three days, you can choose the shortest cycle you’ve had in the last six months to figure out when to start testing.
- Once you’re ready to begin testing, count two weeks out from when your period starts — this should be when ovulation generally occurs. Since ovulation happens a couple of days after the LH surge, you’ll want to test once a day in the couple of days preceding ovulation.
- Also, make sure to note the instructions on the test package. Most tests recommend testing on the same time of day everyday.
Basal Body Temperature
- Aside from self-testing cervical mucus or using an ovulation test kit, one other way to check for ovulation is by measuring basal body temperature (BBT), which is your body’s temperature at rest.
- Believe it or not, a woman’s BBT actually decreases right before her ovary releases an egg. Then, after ovulation, BBT increases by ½ to 1 degree. A woman’s average BBT is around 97–97.5 degrees Fahrenheit before ovulation, and about 97.6–98.6 degrees Fahrenheit afterwards.
- To measure BBT, women can simply take their temperature every day, first thing in the morning, before sitting up. Since this jump in temperature is fairly small, you can buy a specialized basal thermometer that shows the temperature to the tenth of a degree.
- The drawback to this method is: since body temperature only rises after ovulation, by the time you detect the temperature rise, you will have already ovulated. BBT is a good method for women with generally regular cycles who want to track it and notice patterns, but it won’t help you time sex according to ovulation in the moment.
With all of these methods, though, it’s important for women to remember: your body isn’t clockwork, and your cycle might be a little unpredictable, especially if you’re sick, stressed, traveling, or have anything unusual going on. Don’t be too hard on yourself if you’re having trouble tracking your fertility or getting pregnant on a schedule. It’s hard work, and there’s a lot to keep track of!
Troubleshooting
But what if you’ve been tracking your cycle for a while and are still not conceiving?
For women under 35, infertility is defined as not becoming pregnant after one year of regular sex without the use of contraception. If you’re having trouble conceiving, you’re not alone. One in eight couples will have trouble conceiving or keeping a pregnancy.
Timing your sex correctly during the fertile window will increase your chances of getting pregnant. But there are other factors within your control that can improve your chances of getting pregnant like making small improvements to diet and lifestyle.
Men and women should incorporate foods that are high in iron, like spinach and lentils.
For women, being overweight or underweight can also impact fertility and make it difficult to conceive. A healthy body mass index (BMI) is considered anywhere between 18.5 and 24.9, but everyone’s bodies are different, and BMI isn’t the be-all and end-all determinant in healthy weight. Checking in with a physician or nutritionist can help you set realistic, attainable weight loss or weight gain goals.
Steering clear of binge drinking and hard drugs is a generally good lifestyle choice, but especially if you’re trying to get pregnant. Although the research on nicotine’s effect on fertility is somewhat inconclusive, it’s best to cut out cigarettes as well — they can have serious implications for pregnancy.
Got All That?
It’s a lot to keep track of, we know. Understanding how bodies work and tracking their every minute change can feel like a return to high school sex ed, or can make you feel like a science experience, but just remember that you’re doing all of this for a really exciting reason.
And if things aren’t immediately going according to plan, don’t fret. Getting pregnant isn’t as simple as having lots of sex — the sex has to be timed well, both men and women have to make sure to live a healthy lifestyle by all measures, and even then it can still take a while. For those of us going through this journey with advanced reproductive technology like intrauterine insemination or gestational carriers, there can be other ups and downs. We recommend checking in with a physician before you get started and if you’re facing roadblocks. It’s normal for getting pregnant to be a bit of a hustle, and seeking support is so often part of the process.
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